An often used procedure in the cryogenic air separation art is the use of reversing heat exchangers to clean and cool the incoming feed air. In this procedure incoming feed is cooled by indirect heat exchange with one or more outgoing product or waste streams and simultaneously high boiling impurities in the feed, such as water and carbon dioxide, are deposited onto the heat exchanger walls. Before the solid deposits foul the heat exchanger, the feed air stream is switched to a second passageway of the heat exchanger and a waste stream or a product stream which can accomodate the impurities is passed through the passageway containing the deposits, causing these impurities to vaporize and be carried out with this sweep stream. The procedure is repeated when the second passageway has significant deposits on its walls and so on back and forth so that incoming feed air is constantly being cooled and cleaned and the heat exchanger also cleaned of the deposited impurities.
The amount of impurities that the sweep stream can remove from the heat exchanger is dependent on the sweep stream flow rate, pressure level, and temperature relative to the air feed conditions. Since the sweep stream flow rate and pressure are usually set by clean and dry product requirements and separation column pressure levels, the sweep stream temperature relative to the feed air temperature is generally used as the control parameter.
In order to insure that the reversing heat exchanger be self-cleaning as described above it is important that the cold-end temperature differential between the feed stream and the sweep stream be small. Typically, the cold-end temperature difference should be between 0.5 to 2.degree. K. One often used method of attaining this small temperature differential is to pass a stream partially through the heat exchanger thus warming this stream by indirect heat exchange with the feed. This stream, often termed the reversing heat exchanger unbalance stream, is removed from the heat exchanger before it can completely traverse the heat exchanger. This warmed unbalance stream may then be expanded so as to generate refrigeration which is used in the cryogenic air separation plant. One source of the unbalance stream and subsequently expanded stream is the nitrogen-rich vapor from a high pressure cryogenic air separation column. Such vapor is often termed the shelf vapor because historically a shelf was placed near the top of a column to catch reflux liquid. The liquid at this point was often called the shelf liquid and the vapor at this point was often called the shelf vapor.
In small or medium size cryogenic air separation plants the flow requirement for sufficient reversing heat exchanger temperature control is essentially equivalent to the flow requirement for plant refrigeration and therefore the entire warmed unbalance stream is passed through an expander. However for large cryogenic air separation plants, such as those which supply oxygen for coal conversion plants, the flow requirement for reversing heat exchanger temperature control exceeds the requirement for plant refrigeration. This mismatch becomes quite apparent in a plant designed to produce about 1500 tons per day of oxygen although the mismatch can occur at plant sizes as low as 1000 tons per day of oxygen or less depending on how well the plant is insulated.
The mismatch occurs because the flow requirement for reversing heat exchanger temperature control is independent of plant capacity. It is instead dependent on the degree of warming which is required as the stream partially traverses the reversing heat exchanger. That is, the reversing heat exchanger unbalance stream flow requirement for any given unbalance stream temperature increase is a relatively constant percentage of the feed air flow. As the absolute feed air flow increases the absolute unbalance stream flow also increases but the relationship between the two remains essentially constant. The lesser the unbalance stream is warmed the greater is the flow requirement as a percentage of feed air flow and conversely the greater the unbalance stream is warmed the lesser is the flow requirement as a percentage of feed air flow.
However the plant refrigeration requirement is not independent of plant capacity. There are essentially three causes of cold loss in cryogenic plants. Two of these, the net heat input at the warm end of the reversing heat exchanger due to the requirement of temperature differences for heat exchange between incoming feed air and outgoing return streams, and, cold loss related to the loss from the system of liquid water in the operation of the reversing heat exchanger which entered the system as water vapor, vary directly with the incoming feed air flow rate and thus the flow rate of the stream required to generate refrigeration to compensate for these cold losses remains relatively constant as a percentage of the feed air flow. However the third source of cold loss, heat leak into the cryogenic plant from the ambient air, is a function of the surface area of the cryogenic equipment. As is well known the surface area of vessels or conduits generally increases at less than a one to one relationship with the increase in its capacity. Thus cold loss due to heat leak into a cryogenic air separation plant from the ambient air is a smaller amount relative to feed air flow as plant size increases. Thus the flow requirement for plant refrigeration as a percentage of incoming air flow decreases as the plant capacity increases. With the flow requirement as a percentage of incoming feed air flow for the reversing heat exchanger unbalance stream remaining constant as plant capacity increases, the flow mismatch between the unbalance stream requirement and plant refrigeration requirement manifests itself. All other things being equal, the greater is the plant capacity the greater is the flow mismatch. Although one could expand the entire unbalance stream for a large gas plant and thereby generate excess plant refrigeration, this introduces a thermodynamic inefficiency into the system because that exessive refrigeration would have to be degraded needlessely, as for example, by excessive warm end heat exchanger temperature differences.
The above-described mismatch is recognized in the art. One solution disclosed in U.S. Pat. No. 3,947,259-Frischbier divides the warmed unbalance stream and expands only that portion which is required for plant refrigeration. The other portion is liquified, subcooled, expanded and introduced into a low pressure column. This process is disadvantageous because it requires a two-section main condenser with its attendant complexity.
As previously mentioned one important use of large capacity cryogenic air separation plants is to supply process gases to coal conversion plants. Often such plants require, in addition to oxygen, some high pressure nitrogen, as for example, for inert gas blanketing of equipment. Therefore it would be desirable to have a cryogenic air separation process which would solve the large plant flow mismatch problem described above and also efficiently provide high pressure nitrogen.
It is therefore an object of this invention to provide an improved cryogenic air separation process.
It is another object of this invention to provide an improved cryogenic air separation process which eliminates the flow mismatch between unbalance stream and plant refrigeration requirements in large plants while avoiding complex mechanical requirements.
It is a further object of this invention to provide an improved cryogenic air separation process which eliminates the flow mismatch between unbalance stream and plant refrigeration requirements in large plants while also efficiently producing high pressure nitrogen.